Prostheses for implantation in the human body have a substantial size and shape intended to augment or replace body tissues. Because they do not react unfavorably with human tissue, medical grade silicones are the preferred material of construction. Generally a prosthesis has an envelope which holds a deformable material which with the envelope simulates the properties of the tissue that it replaces or augments. Known such materials are silicone gels, and normal saline solution. Obviously the envelope material must be impervious or at least only semi-permeable to what it contains so it does not readily leach or leak into the body. It is general practice for these envelopes to have a smooth outer surface.
There is a growing body of belief to the effect that smooth envelope surfaces have the tendency to become encapsulated in a hard encapsulation developed by the body when it recognizes the implant as a foreign body. Especially in the field of mammary implants, these spherical encapsulations can become individual bodies that are separately palpable, and are quite hard. This totally frustrates the intended purpose of the prosthesis, and quite frequently requires a revision of the procedure. This revision means the implantation of another prosthesis. In such a situation, everybody loses. The patient is distressed and subjected to the risks and expenses of a second procedure, and the surgeon is subjected to attendant risks and client displeasure.
A line of prostheses has been developed which is covered by a material with a roughened surface. This surface has a wealth of cavities and fibrous structures. It is generally an open cell foam, cleanly cut to make this surface texture. The material used for this purpose is usually a urethane open cell foam. It would be preferred to use a silicone foam for this purpose, but known efforts to provide silicone foams have not provided a foam structure which the inventor herein regards as suitable. The terms "foam" and "sponge" are used interchangeably herein to denote an open cell construction wherein cavities in the material interconnect with one another.
It is an object of this invention to provide a silicone foam and a method to make it, and especially to form it on the surface of a continuous substrate so the resulting construction has an inherent and continuous outer roughened surface region, and an interior layer which is impervious or at least only semi-permeable, to what it must contain.
It is unimportant to the invention that the cellular construction extend all the way through the material, but this is possible to make if only a foam is desired. However in the field of implants it is the surface configuration with cavities and fibrous material which is important to surrounding tissue. A depth of perhaps no more than about 1 millimeter of such texture, is of consequence. Much deeper cellular construction can itself lead to problems of tissue growth.
The inventor herein has found that by embedding soluble crystals in the silicone to a desired depth to which the foam will be produced, curing the silicone, and then dissolving the crystals, an open-foam cell can be made which provides the roughened surface desired for limited tissue ingrowth. The foam can be formed as a surface on an integral and continuous silicon substrate, the depth of the foam being controllable by controlling the depth to which the crystals extend.
The inventor has further found that the use of crystals all of substantially the same size results in a relatively less useful foam. This is because particles of equal size pack together to form a matrix of relatively low percentage of volume ("packing ratio"). If more than one size is used, then a less dense foam can be made, because more crystals can be packed into a given volume. Furthermore, when dissolved out, there is likely to be a greater number of tendrils and interconnecting open cavities, which provides a correspondingly more effective roughened surface.
It is an object of this invention to provide a silicone foam, a silicone body surfaced with an integral silicone foam, and a method for making them.